Materials that are permeable to water only in vapor form have been produced for a considerable time. These materials do not allow the passage of water in liquid form. This material is extremely useful where it is desired to keep underlying articles or skin dry while allowing air and water vapor to travel through the material. These materials are useful for athletic clothing, medical garments, rain gear, tarpaulins, tents and other waterproof garments.
It is often desirable to fasten these materials to other substrates. Polyurethanes are commonly used as adhesives or coatings on such material. It is therefore necessary that the polyurethane composition also have a high moisture vapor transmission rate.
U.S. Pat. No. 4,194,041 to Gore et al. issued Mar. 18, 1980 teaches a layered article that prevents liquid water from penetrating through while at the same time permitting moisture vapor to pass out through the article. Gore teaches a hydrophilic layer which is a liquid polyether-polyurethane. The liquid polyurethane involves the mixing of two or more liquids, which are generally of low molecular weight. The initial physical properties of the system are poor until the curing proceeds to some degree. Such characteristics include green strength and low flow characteristics. Green strength refers to the strength of the bond after the composition sets, but before the composition is cured. Generally, these polyurethane adhesives have value where the initial strength of the bond is unimportant. Substantial bonding often forms over a period of time between hours and days and the resultant bond has high tensile strength and is strong and resilient to external forces.
U.S. Pat. No. 4,925,732 to Driskill et al. teaches a laminate which comprises a solvent based moisture permeable adhesive. This adhesive is the reaction product of a polyol of primarily oxyethylene units, a chain extender having a molecular weight in a range lower than about 500 and a polyisocyanate. These adhesives are undesirable because of the problems typically associated with the use of a solvent.
U.S. Pat. No. 4,532,316 to Henn issued Jul. 30, 1985 teaches how to make and use a hot melt moisture cure polyurethane prepolymer with high moisture vapor transmission rates which in elastomeric film form may be used in combination with other materials useful in rain protective garments. Henn uses a polyol, a polyisocyanate and a chain extender having a molecular weight in a range lower than about 500 used to build molecular weight faster, and therefore increase the green strength. Henn exemplifies the use of a poly(oxyethylene) glycol. There are disadvantages to the use of chain extenders including lower melt rates caused by the use of the chain extender, poor green strength and longer open times which consequently results in a surface which remains tacky longer. For coating applications, production will be slowed down, or it may be necessary to transfer coat which involves more steps. The transfer coating process also prevents the polyurethane composition from having an exposed surface during manufacturing which could result in the polyurethane undesirably adhering to equipment, or to other surfaces. Another problem associated with the use of chain extenders is stability during storage and during use at typical application temperatures from about 90.degree. C. to about 110.degree. C. Gelling is usually an indication of instability and can result when too much crosslinking takes place, ultimately destroying the processability and applicability of the prepolymer. In Example 1 at column 21 lines 29-32, Henn refers to a storage stability of 4 months, and in Example 3 at column 25 lines 29-31, Henn refers to the packaged prepolymer as remaining workable for at least several days. While this hot melt moisture cure is an improvement over the use of liquid moisture cure polyurethanes, the rate of set and green strength are still undesirably slow.
U.S. Pat. No. 5,508,371 to Werenicz et al. issued Apr. 16, 1996 teaches a polyurethane composition with superior moisture vapor transmission. Werenicz et al. teaches the use of a polyester polyol segmented with a polyether. Specifically, polyester polyols built up from aliphatic and aromatic dicarboxylic acids and diols having a chain length of between C.sub.2 to C.sub.20. The OH number of the polyester group lies between 10 and 50, and preferably between 10 and 40.
U.S. Pat. No. 5,166,302 to Werner et al. teaches a moisture curing polyurethane hot melt adhesive which utilize crystalline polyester polyols which have short setting times. However, Werner et al. does not teach nor suggest how to combine specific crystalline polyester polyols with specific polyether polyols to obtain rapid setting hot melt moisture cure compositions which have high moisture vapor transmission rates. In fact, Werner et al. teaches moisture-crosslinking hot melt adhesives which are highly resistant to the creepage of moisture as found at column 1 lines 64 to 68 and column 2 lines 1 to 2.
The present inventors have found a unique combination of polyols and isocyanates which results in prepolymers that have low viscosity, excellent thermal stability and a rapid set time as well as excellent green strength without the use of a chain extender. Green strength refers to the strength of a bond after the composition sets, but before the composition is cured. Once cured, these one-part hot melt moisture cure polyurethane compositions have surprisingly high green strength, very good flexibility, excellent mechanical strength and an excellent moisture vapor transmission rate.